The discovery boards are absolutely awesome and very cheap. I highly recommend them as well!

There is also a fantastic RTOS called ChibiOS that has a focus on the STM32 chips and has great support.

ChibiOS seems to work OK for me, though I haven't fully tested it yet. I'm running a port on both the M4 and M0 cores of an LPC4357, the 204mHz part. It's pretty quick.